Various types of systems for heating and/or cooling various facilities have been developed. One type of system utilizes heat collected from natural sources or excess heat produced as a byproduct of other processes. Such systems typically include a heat source such as solar collectors, a heat-using facility, and a thermal storage unit. The thermal storage unit can be utilized to compensate for time differences between peak thermal energy generation and thermal energy use requirements. For example, if solar energy is being utilized to heat buildings or the like, solar collectors may be utilized to collect heat. However, such collectors typically collect the most thermal energy during the day, whereas a building may need to be heated at night when the ambient air is colder. The thermal storage unit stores excess energy collected during periods of time during which thermal energy collection exceeds thermal energy use, and the stored thermal energy is drawn from the thermal energy storage unit during periods of time during which the thermal energy needs exceed thermal energy collection/production.
One type of thermal storage system utilizes hot oil that is stored in a large tank that may be surrounded by insulating material. During periods of time when the heat being collected exceeds the heat being used, the oil is heated by a solar collector or other heat source, and the hot oil is pumped into the tank. Cooler oil may be drawn from another portion of the tank that is generally remote from the hot oil inlet. The cooler oil is circulated through the solar collectors or other heat source, and the heated oil is then returned to the tank. During times when the amount of heat needed exceeds the amount of heat being generated, hot oil from the tank is pumped to a building or other facility that uses heat. The hot oil may pass through a heat exchanger where the heat is removed from the oil, and the cooled oil is then returned to the tank.
Another type of thermal storage system utilizes molten salt or a hybrid hot oil/molten salt system. Typically, a mixture of 60% sodium nitrate and 40% potassium nitrate is used. This mixture melts at a temperature of 429° to 430° F. The systems are typically kept at a minimum temperature of 550° F. on the cool side, and can be heated to 1100° F. or more. Such systems may include two tanks wherein a “cold” tank is held at 550° F. and a “hot” tank is kept as high as possible within the limitations of the equipment.
Another type of thermal storage system includes hot water tanks. Hot water from solar collectors or other source is pumped into the tanks, and the heated water is pumped from the tank at a later time when the thermal energy is needed. Such systems suffer from limitations. For example, such systems cannot be utilized to raise the temperature of the water above the boiling point unless the water is maintained at a high pressure. However, providing equipment capable of storing water at high pressures may be difficult, and it may raise safety concerns as well.